The disaggregated configuration can be set at both the connection level and the
table level. For example:

# Connection-level: the page log.
conn = wiredtiger_open("disaggregated=(page_log=palite,...)")

# Table-level: the storage tier.
session.create("layered:test", "disaggregated=(storage_tier=cold)")

One may assume both disaggregated fields make it into the layered metadata: the
page_log from the connection and the table's own storage_tier.

However, test_layered_schema31.py shows this is not the case. The page_log is omitted.

The root cause is as follows:

In __create_layered, the configuration across the connection and the table is placed
in the variable layered_cfg:

layered_cfg[1] = "disaggregated=(page_log=whatever)"; // connection
layered_cfg[2] = "disaggregated=(storage_tier=cold)"; // table

__wt_config_collapse, called in __create_layered, takes settings at higher
layered_cfg indices over lower ones. What it should do instead is merge the inner
fields of disaggregated across both levels.

Note that despite page_log being omitted from the metadata, the rest of the code copes
with a fallback like this:

if (ret == WT_NOTFOUND || page_log_item.len == 0)
    npage_log = S2C(session)->disaggregated_storage.npage_log;